The case for a dynamic contact angle in containerless solidification

Containerless solidification, in which the melt is confined by its own surface tension, is an important technique by which very pure materials can be produced, The form of the solidified product is sensitive to conditions at the tri-junction between the solid, the melt and the surrounding vapor. An understanding of the dynamics of tri-junctions is therefore crucial to the modelling and prediction of containerless solidification systems. We consider experimentally and analytically the simple system of a liquid droplet solidifying on a cold plate. Our experimental results provide a simple test of tri-junction conditions which can be used in theoretical analyses of more complicated systems. A new dynamical boundary condition at the tri-junction is introduced here and explains the surprising features of solidified water droplets on a cold surface.